Odd issue with Stongswan and Throughput

I recently got a Linksys WRT 1900ACS and first thing i did was install OpenWRT i love the software and the community.

I've got an odd problem where iPerf3 from 1 site to another site works fine, but i have slow throughput one way but only from a client machine connected to the 1900ACS. If i Run iPerf3 on the router i get good throughput.

for example

Site 1 server 1 -> Sophos XG ( -------ipsec----->Linksys WRT1900ACS ( =36Mbps

Site 1 server 1 -> Sophos XG ( -------ipsec----->Linksys WRT1900ACS ( -> Site 2 Server 2 = 36Mbps

Linksys WRT1900ACS ( -------ipsec-----> Sophos XG ( -> Site 1 server 1 = 50Mbps

The problem occurs when running iPerf from a server or client behind the Linksys router and its sending data ;

Site 2 Server 2 ->Linksys WRT1900ACS ( -------ipsec-----> Sophos XG ( -> Site 1 server 1 = 3Mbps

Site 1 is Virgin Media running at 1500MTU
Site 2 is BT FTTP PPPoE running at 1500MTU (Baby Jumbos)

As the through put is fine from the OpenWRT Router i'm assuming the ipsec tunnel is correctly configured and MTUs are all good.

I Should also mention that Site 1 has a IPSec connection to Azure and Iperf to a virtual machine hosted there works fine.

Routing and NAT appears to be okay as everything is working just a bit slow as described.

I'd appreciate any help or pointers anyone can give.

Okay so looking into this all seems fine. The oddity is when Windows server 2016/2019 or windows 10 is used. Running iperf from Server 2012R2 and also linux distros works fine....

So now i just need to find whats causing the issue in windows 10/2016